1. Field of the Invention
The present invention relates to an assembly method for a nuclear fuel assembly, capable of preventing scratches from occurring on outer peripheral surfaces of nuclear fuel rods when the nuclear fuel assembly is assembled, and to a grid in the nuclear fuel assembly for supporting the fuel rods.
2. Prior Art
FIG. 10 of the attached drawings depicts an example of a nuclear fuel assembly which is mounted on a nuclear reactor such as a pressurized water reactor or the like.
In this assembly, a pair of top and bottom nozzles 1 and 2 are arranged in facing relation to each other and in vertically spaced relation to each other. A plurality of control-rod guide thimbles 3 extend between and are securely fixed to the top and bottom nozzles 1 and 2. A plurality of grids 4 are secured to intermediate portions of the respective control-rod guide thimbles 3 in vertically spaced relation to each other.
As shown in FIGS. 11 through 13, each of the grids 4 is formed as follows. That is, a plurality of straps 7, each in the form of a thin strip sheet, are assembled perpendicularly to each other into a grid by mutual fitting of slits 8 which are formed in the straps 7 in longitudinally equidistantly spaced relation to each other. A plurality of grid cells 5 are defined in each of the grids 4. A pair of dimples 9 and 9 and a pair of springs 10 and 10 for supporting a fuel rod 6 are formed on the wall surface of each of the grid cells 5 in opposed relation to each other. The fuel rod 6 inserted in the grid cell 5 is supported in urging relation to the dimples 9 by the springs 10.
An assembly method of the nuclear fuel assembly constructed above will next be described.
First, the grids 4 are arranged in vertically spaced relation to each other at a predetermined spacing. The control-rod guide thimbles 3 are then inserted into and fixed to predetermined grid cells 5 of each of the grids 4, respectively. Subsequently, the fuel rods 6 are inserted into corresponding grid cells 5 in each of the grids 4 which are supported by the control-rod guide thimbles 3, with the fuel rod 6 in sliding contact with the dimples 9 and the springs 10. In this manner, the fuel rods 6 are fixedly arranged in corresponding grid cells 5 through the dimples 9 and the springs 10. After insertion of all the fuel rods 6, the pair of top and bottom nozzles 1 and 2 are fixedly mounted respectively to the opposite ends of the control-rod guide thimbles 3.
In the aforesaid assembly method of the nuclear fuel assembly, when the fuel rods 6 are inserted respectively into the grid cells 5 in each of the grids 4, the outer peripheral surface of each of the fuel rods 6 is clamped between the dimples 9 and the springs 10, and the fuel rod 6 must be inserted into the corresponding grid cells 5, while the fuel rod 6 resists the resilient force of the springs 10. Thus, there is a problem that the outer peripheral surface of the fuel rod 6 is scratched by the springs 10 and the dimples along its longitudinal direction.